1. Technical Field
This invention relates to a method and means for restoring the continuity of the circuit of two or more series connected filament lamps whenever one lamp filament opens.
2. Description of Related Art
Lamps that form a part of an extensive array, such as airport guidance and runway lights, are frequently arranged in small groups of two or more that are series connected. That arrangement is advantageous as it is more economical to install than is a parallel connected lamp array. However, the series connected lamp groups have the disadvantage that if the filament of one lamp in the group fails, the other lights within the group go dark.
It is known in the prior art to use a solenoid in combination with a toggle switch, or a circuit breaker in combination with a micro switch, to bridge the opened lamp filament and thereby maintain operation of the remaining lamps in the group. That approach can only be used with a constant current source supplying the lamp group otherwise the remaining lamps within the group will be subjected to a damaging over-voltage. The controllers for constant current sources are slow to respond, or ramp down, to the preset current level when a portion of the resistive load, in this case a lamp filament, is suddenly removed. That slow response time subjects the remaining lamps within a group to over-voltages of significant magnitude that may last for several seconds. For example in a three-lamp group as one filament is bridged, the over-voltage jumps to approximately 150% of the rated lamp voltage and then decreases to the rated lamp voltage as the controller ramps down.
Advantages provided by use of this invention include the prevention of over-voltage damage to a series connected group of filament lamps by instantly substituting a resistor in place of an opened lamp filament; allowing the use of a constant voltage rather than a constant current source; and obtaining substantial cost savings.